Target fec (forwarding equivalence class) stack based fec query in segment routing environments

ABSTRACT

In one embodiment, a method includes generating a first trace request at an initiator node configured for segment routing, the first trace request comprising a query for FEC (Forwarding Equivalence Class) information, transmitting the first trace request on a path comprising at least one node wherein FEC details for the node are unknown by the initiator node, receiving a response to the first trace request comprising the FEC information, transmitting a second trace request with the FEC information, and receiving a response to the second trace request providing FEC validation. An apparatus is also disclosed herein.

STATEMENT OF RELATED APPLICATION

The present application claims priority from U.S. ProvisionalApplication No. 62/365,336, entitled TARGET FEC (FORWARDING EQUIVALENCECLASS) STACK BASED FEC QUERY IN SEGMENT ROUTING ENVIRONMENTS, filed onJul. 21, 2016 (Attorney Docket No. CISCP1308+). The contents of thisprovisional application are incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates generally to communication networks, andmore particularly, to connectivity verification in Segment Routingnetworks.

BACKGROUND

Segment Routing (SR) architecture leverages source routing and tunnelingparadigms and can be directly applied to an MPLS (Multiprotocol LabelSwitching) data plane. A node steers a packet through a controlled setof instructions called segments, by prepending the packet with an SRheader. Rather than depending on a hop-by-hop signaling technique, SRdepends on a set of segments that are advertised by a routing protocol.These segments act as topological sub-paths that can be combinedtogether to form a desired path. Segment Routing allows a head end ofthe path to impose a stack of labels without knowing the control planedetails of each of those labels. This raises challenges in pathvalidation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a network in which embodimentsdescribed herein may be implemented.

FIG. 2 illustrates another example of a network in which embodimentsdescribed herein may be implemented.

FIG. 3 depicts an example of a network device useful in implementingembodiments described herein.

FIG. 4 is a flowchart illustrating a process for FEC stack based query,in accordance with one embodiment.

FIG. 5 illustrates a trace request generated at an initiator node intrace mode, in accordance with one embodiment.

FIG. 6 illustrates a response validating an FEC in the first datacenter.

FIG. 7 illustrates a trace request with an FEC query and a traceresponse including FEC details for a node in an MPLS cloud.

FIG. 8 illustrates a trace request with the FEC query replaced with theFEC received in the response of FIG. 7.

FIG. 9 illustrates another response providing FEC details of a nextlabel in the MPLS cloud.

FIG. 10 illustrates a trace request with the FEC query replaced with theFEC received in the response of FIG. 9.

FIG. 11 illustrates FEC validation in the MPLS cloud in response to thetrace request in FIG. 10.

FIG. 12 illustrates a trace request and a response providing FEC detailsfor a node in a second data center.

FIG. 13 illustrates another trace request in which the FEC query wasreplaced with the FEC received in the response of FIG. 12.

FIG. 14 illustrates FEC validation in the second data center in responseto the trace request shown in FIG. 13.

FIG. 15 illustrates a trace request and a response from an egress nodevalidating the top FEC in an LSP.

FIG. 16 illustrates a trace packet generated at an initiator node inping mode, in accordance with one embodiment.

FIG. 17 illustrates a trace response providing FEC details for an egressnode.

FIG. 18 illustrates a trace request containing the FEC details from theresponse in FIG. 17.

FIG. 19 illustrates a trace response from an egress node validating theFEC.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

In one embodiment, a method generally comprises generating a first tracerequest at an initiator node configured for segment routing, the firsttrace request comprising a query for FEC (Forwarding Equivalence Class)information, transmitting the first trace request on a path comprisingat least one node wherein FEC details for the node are unknown by theinitiator node, receiving a response to the first trace requestcomprising the FEC information, transmitting a second trace request withthe FEC information, and receiving a response to the second tracerequest providing FEC validation.

In another embodiment, an apparatus generally comprises an interface fortransmitting packets on an LSP (Label Switched Path), a processor forgenerating a trace request at an initiator node configured for segmentrouting, the trace request comprising a query for FEC (ForwardingEquivalence Class) information, transmitting the trace request on a pathcomprising at least one node wherein FEC details for the node areunknown by the initiator node, and processing a response to the tracerequest comprising the FEC information.

In yet another embodiment, logic is encoded on one or morenon-transitory computer readable media for execution and when executedoperable to generate a first trace request at an initiator nodeconfigured for segment routing, the first trace request comprising aquery for FEC (Forwarding Equivalence Class) information, transmit thefirst trace request on a path comprising at least one node wherein FECdetails for the node are unknown by the initiator node, process aresponse to the first trace request comprising the FEC information forthe node, transmit a second trace request with the FEC information, andprocess a response to the second trace request providing FEC validation.

EXAMPLE EMBODIMENTS

The following description is presented to enable one of ordinary skillin the art to make and use the embodiments. Descriptions of specificembodiments and applications are provided only as examples, and variousmodifications will be readily apparent to those skilled in the art. Thegeneral principles described herein may be applied to other applicationswithout departing from the scope of the embodiments. Thus, theembodiments are not to be limited to those shown, but are to be accordedthe widest scope consistent with the principles and features describedherein. For purpose of clarity, details relating to technical materialthat is known in the technical fields related to the embodiments havenot been described in detail.

Path validation may be used to verify end-to-end connectivity along adata path. One mechanism for detecting data plane failures in LSPs(Label Switched Paths) is modeled after a ping/traceroute paradigm andreferred to as LSP Ping. LSP Ping traditionally uses a target FEC(Forwarding Equivalence Class) stack for FEC validation. The LSP to betested is identified by the FEC stack. The test verifies that packetsthat belong to a particular FEC actually end their path on a node thatis an egress for that FEC. LSP Ping machinery defined in IETF (InternetEngineering Task Force) RFC (Request for Comments) 4379 (“DetectingMulti-Protocol Label Switched (MPLS) Data Plane Failures”, K. Kompellaet al., February 2006) carries FEC details in a Target FEC Stack Sub-TLV(Type-Length-Value) that may be used by transit or egress nodes for FECvalidation.

Segment Routing (SR) leverages source routing and tunneling paradigmsand can be directly applied to an MPLS (Multiprotocol Label Switching)data plane. Segment Routing differs from other MPLS control planeprotocols, in that segment assignment is not on a hop-by-hop basis.Segment Routing allows a head end to impose a stack of labels withoutknowing the control plane details of each of those labels. This raisesissues for connectivity verification in an LSP with segment routingarchitecture.

IETF draft-ietf-mpls-spring-lsp-ping-00 (“Label Switched Path (LSP)Ping/Trace for Segment Routing Networks Using MPLS Dataplane”, N. Kumaret al., May 10, 2016) proposes an extension for LSP Ping with SegmentRouting that utilizes different FECs (IPv4 IGP-Prefix Segment ID, IPv6IGP-Prefix Segment ID, and IGP-Adjacency Segment ID). These mechanismsmay not provide sufficient validation when the FEC being tracedtraverses one or more MPLS tunnels or when LSP stitching is used. Forexample, in some scenarios an initiator of a connectivity test may nothave any way to fetch FEC details. Thus, conventional LSP Ping may notprovide FEC validation in Segment Routing networks due to lack ofdownstream FEC details.

FEC stack changes proposed in IETF RFC 6424 (“Mechanism for PerformingLabel Switched Path Ping (LSP Ping) over MPLS Tunnels”, N. Bahadur etal., November 2011) may not be sufficient to provide FEC validation overone or more MPLS tunnels. As per RFC 6424, when a transit node PUSHes anew label (LSP stitching, LSP tunneling, hierarchical LSP etc.), itincludes the new FEC details. When it is a POP, it just includes thedetails about the label to be popped. In Segment Routing, a responsefrom a responder will carry FEC stack change (e.g., as described in IETFRFC 6424) and instruct an ingress node to POP the top FEC in the TargetFEC Stack. The response may also include downstream details (e.g.,interface address and router ID). However, there is no mechanism toinstruct the downstream FEC details in the label stack.

Additionally, it may be desired to stitch the SR control plane withtraditional MPLS control planes running LDP (Label DistributionProtocol) and/or RSVP-TE (Resource Reservation Protocol)-(TrafficEngineering). An operator running Segment Routing may push a label stackto validate the end-to-end LSP. However, a provider edge node may nothave knowledge of the MPLS cloud to specify the corresponding FECdetails in the Target FEC Stack (TFS). The conventional response frominstructing FEC stack change is not sufficient to allow inclusion ofdownstream FEC details from the MPLS cloud.

A similar use case is a network in which the operator desires to stitchtraditional MPLS LDP and RSVP-TE LSPs with Segment Routing LSPs runningBGP (Border Gateway Protocol) and/or IGP (Interior Gateway Protocol). Anoriginator in a Segment Routing cloud may not have intimate knowledge ofthe MPLS cloud to include in the TFS.

As described above, conventional mechanisms may not be sufficient toprovide end-to-end connectivity verification when an LSP traverses oneor more tunnels or LSP stitching is used, for example, in SegmentRouting networks.

The embodiments described herein may be used to provide LSP Ping controlplane validation for Segment Routing (SR), as well as for SR<->LDP(Label Distribution Protocol) hybrid deployments and scenarios. Aspreviously described, a Segment Routing head end may not have thecontrol plane details of the label of each label stack element in theSegment Routing (or SR+LDP) stack. Consequently, it cannot fill in theTFS (Target FEC Stack). The embodiments described herein provide a meansfor obtaining FEC details and control plane information from a nodeadjacent to the node with context for the label. In one or moreembodiments, a new semantic is provided for the Target FEC Stack (TFS),so that the target is a query. An adjacent node may then be queried forcontrol plane details of the downstream node.

Referring now to the drawings, and first to FIG. 1, a network in whichembodiments described herein may be implemented is shown. Forsimplification, only a small number of nodes are shown. The embodimentsoperate in the context of a data communication network includingmultiple network devices. The network may include any number of networkdevices in communication via any number of nodes (e.g., routers,switches, gateways, controllers, edge devices, access devices,aggregation devices, core nodes, intermediate nodes, or other networkdevices), which facilitate passage of data within the network. Thenetwork devices may communicate over one or more networks (e.g., localarea network (LAN), metropolitan area network (MAN), wide area network(WAN), virtual private network (VPN), virtual local area network (VLAN),wireless network, enterprise network, corporate network, data center,Internet, intranet, radio access network, public switched network, orany other network).

In the example shown in FIG. 1, two data centers 10, 12 (Data Center 1,Data Center 2) and an MPLS cloud 14, each include a plurality of nodes.Data Center 1 includes vPE1 (virtual Provider Edge 1) (10 a), ToR1 (Topof Rack 1) (10 b), and Spine1 (10 c). MPLS Cloud 14 includes PE1 (14 a),P2 (14 b), P3 (14 c), and PE4 (14 d). Data Center 2 includes Spine2 (12a), ToR2 (12 b), and vPE2 (12 c). The nodes may comprise routers or anyother network device configured to perform routing functions. Eachnetwork may include any number of edge routers (e.g., PE nodes), corerouters (e.g., P nodes), or virtual routers (e.g., vPE).

One or more nodes are configured to perform Segment Routing, whichspecifies a path that a packet will take through the network using astack of segment identifiers (SIDs). A node steers a packet through acontrolled set of instructions called segments by prepending the packetwith a Segment Routing header. Segment Routing may be directly appliedto an MPLS data plane.

Packets may be transmitted within the network via a Label Switched Path(LSP). Packets may enter the network via an ingress node, travel alongan LSP of one or more core (transit) nodes and exit via an egress node.Each LSP is defined by a set of labels, which comprise locallysignificant identifiers that are used to identify a ForwardingEquivalence Class (FEC). The FEC represents packets that share a samerequirement for transport (e.g., over the same path with the sameforwarding treatment). Each LSP is associated with at least one FEC thatspecifies which packets are mapped to that LSP.

As shown in FIG. 1, the network may comprise any number of networks ortunnels. The network may also include stitched LSPs involving two ormore LSP segments stitched together. In order to perform an end-to-endtrace, the ingress node needs to know FEC information regarding each ofthe networks or stitched LSP segments.

A connectivity test may be performed by an ingress node of the LSPacting as an initiator node (e.g., node 10 a in FIG. 1). The initiatornode 10 a initiates the connectivity test to trace and validate thepaths between the ingress node and the egress node. In one embodiment,the connectivity test may be used to detect data plane failures in oneor more LSP by exchanging request and response (reply) messages 17, 19(also referred to as echo request/reply or trace request/replymessages). The trace request 17 may be used to test a particular LSPidentified by an FEC stack and sent along the same path as other packetsbelonging to a given flow. The initiator node 10 a transmits the tracerequest packet 17 and includes information identifying the flow (e.g.,identified by an FEC) in an FEC stack. When the trace request isreceived, the receiver is expected to verify that the control plane anddata plane are both healthy (for the FEC stack being pinged) and thatthe two planes are in sync. The connectivity test is successful when thetrace request is received at an edge node that is verified to be theegress node for the flow identified in the trace request. As previouslydescribed, the initiator node may not have FEC details for each label inthe label stack for the path. Thus, it cannot fill in the Target FECstack and perform connectivity tests for the entire path (LSP, LSPs,segments) without obtaining additional FEC details and control planeinformation.

In one or more embodiments a new Target FEC Stack (TFS) Sub-TLV 16(referred to herein as FEC-Query) is inserted into the trace requestpacket 17 to obtain FEC details needed to verify connectivity. TheFEC-Query queries a node for control plane details on the FEC. Aninitiator node may include the FEC-Query Sub-TLV in a Target FEC Stackfor each label in the stack with unknown FEC. A responder node mayinclude the FEC details associated with the top label in the labelstack. The initiator may then replace the FEC-Query Sub-TLV with the FECdetails received in the response and send out a connectivityverification request. This process may be repeated one or more timesalong a path over any number of networks until the egress node isreached to provide end-to-end connectivity verification in a SegmentRouting network.

FIG. 1 illustrates a label stack 15 comprising a new Target FEC Stack(TFS) Sub-TLV with three FEC-Query entries 16. In this example, the TFSSub-TLV includes a first FEC label for a known FEC (e.g., Spine1 in DataCenter 1) and three FEC-Query entries for PE1, PE4, and vPE2 (nodes(labels) with unknown FEC (FEC details, control plane information)). Thesemantic associated with the TFS Sub-TLV is to have the responder replywith the FEC details of the relevant label in the stack. The TFS sub-TLVmay be used, for example, to obtain the FEC details and control planeinformation from the node adjacent to the one with context for thelabel. As described in detail below, the initiator may include anFEC-Query Sub-TLV in the Target FEC stack for each label in the stackwith unknown FEC. The responder includes the FEC details associated withthe next label in the label stack. The responder may perform FECvalidation by fetching the label from the label stack of the receivedtrace request and FEC details from the Target FEC Stack TLV to validateagainst the local control plane information.

An initiator node (e.g., node 10 a in FIG. 1) may generate the LSP Ping(trace request) at an FEC query module 18. The FEC query module 18 mayinclude initiator logic operable to generate the trace request packet17. For example, the FEC query module 18 may be operable to generate thetrace (echo) request and forward the packet toward an egress of the LSPbeing tested and process or store a trace response (echo reply) 19. Thetrace request 17 may be generated upon receipt of a connectivity testinitiation message or a trace response 19.

The Segment Routing control plane may also be used with traditional MPLScontrol planes running LDP and/or RSVP-TE as shown in the example ofFIG. 2. In this example, two BGP/IGP Segment Routing networks 20, 22 arein communication via an MPLS LDP/RSVP-TE network 24. The networks 20,22, 24 comprise a plurality of Provider Edge (PE) nodes 26 and core(Provider (P)) nodes 28. As previously described, the initiator PE1 maynot have knowledge of the MPLS cloud sufficient to specify correspondingdetails in the Target FEC stack. The TFS sub-TLV with FEC-Query 16 maybe used to obtain the FEC details as described below for the networkshown in FIG. 1.

Details of example embodiments are provided below with respect to FIGS.5-15 (trace mode) and 16-19 (ping mode).

It is to be understood that the network devices and topologies shown inFIGS. 1 and 2, and described above are only examples and the embodimentsdescribed herein may be implemented in networks comprising differentnetwork topologies or network devices, or using different protocols,without departing from the scope of the embodiments. For example, thenetwork may include any number or type of network devices thatfacilitate passage of data over the network (e.g., routers, switches,gateways, controllers, appliances), network elements that operate asendpoints or hosts (e.g., servers, virtual machines, clients), and anynumber of network sites or domains in communication with any number ofnetworks. Thus, network nodes may be used in any suitable networktopology, which may include any number of servers, virtual machines,switches, routers, or other nodes interconnected to form a large andcomplex network, which may include cloud or fog computing. Nodes may becoupled to other nodes or networks through one or more interfacesemploying any suitable wired or wireless connection, which provides aviable pathway for electronic communications.

FIG. 3 illustrates an example of a network device 30 that may be used toimplement the embodiments described herein. In one embodiment, thenetwork device 30 is a programmable machine that may be implemented inhardware, software, or any combination thereof. The network device 30includes one or more processor 32, memory 34, network interfaces 36, andFEC query module 18.

Memory 34 may be a volatile memory or non-volatile storage, which storesvarious applications, operating systems, modules, and data for executionand use by the processor 32. For example, components of the FEC querymodule 18 (e.g., code, logic, database, etc.) may be stored in thememory 34. Memory 34 may also include an SR database, routing table(e.g., routing information base (RIB)), forwarding table (e.g.,forwarding information base (FIB)), or any other data structure for usein routing or forwarding packets. The network device 30 may include anynumber of memory components.

Logic may be encoded in one or more tangible media for execution by theprocessor 32. For example, the processor 32 may execute codes stored ina computer-readable medium such as memory 34. The computer-readablemedium may be, for example, electronic (e.g., RAM (random accessmemory), ROM (read-only memory), EPROM (erasable programmable read-onlymemory)), magnetic, optical (e.g., CD, DVD), electromagnetic,semiconductor technology, or any other suitable medium. In one example,the computer-readable medium comprises a non-transitorycomputer-readable medium. The network device 30 may include any numberof processors 32.

The network interfaces 36 may comprise any number of interfaces(linecards, ports) for receiving data or transmitting data to otherdevices. The network interface 36 may include, for example, an Ethernetinterface for connection to a computer or network.

It is to be understood that the network device 30 shown in FIG. 3 anddescribed above is only an example and that different configurations ofnetwork devices may be used. For example, the network device 30 mayfurther include any suitable combination of hardware, software,algorithms, processors, devices, components, or elements operable tofacilitate the capabilities described herein.

FIG. 4 is a flowchart illustrating an overview of a process for FECquery in Segment Routing networks, in accordance with one embodiment. Asdescribed above, the FEC query may be used in an LSP Ping or othertraceroute, connectivity verification, or fault detection process. Atstep 40, an initiator node (e.g., node 10 a in FIG. 1) generates a tracerequest (e.g., echo request) with at least one FEC query correspondingto a label in an FEC stack with an unknown FEC (e.g., node/label forwhich FEC details and control plane information are unknown). Asdescribed below, the trace request may be configured for operation intrace mode in which the trace request has an incrementing TTL, or pingmode in which the trace request has a TTL in the bottom label set to 1.The initiator transmits the trace request on a path with the unknown FEC(step 42). The initiator node 10 a receives a first response providingFEC information for the node with the unknown FEC (step 44). The FECinformation (details) may comprise, for example, a destination IPsubnet, traffic class, or other identifier associated with a label and aforwarding path and representing a class or category of packets.

The initiator node 10 a may then send another trace request with the FECinformation inserted into the request (step 46). The initiator node 10 athen receives a second response validating the FEC (step 48). Theresponse may provide connectivity verification for a portion (orsegment) of the path or the complete LSP. This process may be repeatedany number of times to obtain FEC details and control plane informationanywhere along the path so that end-to-end connectivity may be verified.

It is to be understood that the flowchart shown in FIG. 4 and describedabove is only an example and that steps may be added, combined, removed,or modified, without departing from the scope of the embodiments. Forexample, rather than using the FEC information for FEC validation, asdescribed above for steps 46 and 48, the FEC information may be used forother means, such as remote learning for controller validation, or anyother function. In one example, steps 46 and 48 may be replaced with astep for transmitting the FEC information to a controller, for use bythe controller.

A detailed example of a process in accordance with one embodiment, fortarget FEC stack based FEC query in the network of FIG. 1 is provided inFIGS. 5-15. This example utilizes a trace mode in which the initiatorincludes an FEC-Query Sub-TLV 56 in the target FEC stack for each labelin the stack with unknown FEC and the responder includes the FEC detailsassociated with the top label in the label stack.

Referring now to the example shown in FIG. 5, vPE1 (initiator node) isstatically configured with a tunnel for vPE2 and is instructed to push{20003;17001;17004;20001} as a label stack. Node vPE1 may not be awareof the FEC details for each label. Therefore, if conventional LSP pingis triggered from vPE1, it would not have sufficient details to use inthe Target FEC stack. As previously noted, this raises challenges invalidating the LSP Path. As described below, the TFS sub-TLV FEC queryis used to query an upstream node for control plane details on the FEC.

In the topology shown in FIG. 5, vPE1 and Spine1 are in the same domain.When vPE1 triggers an LSP trace to vPE2, it includes the Target FECStack and populates the same with FEC-Query Sub-TLV 58 for each label inthe label stack with unknown FEC. The initiator (vPE1) includes an FECquery 58 to FECs corresponding to all segments imposed in the labelstack for which FEC details and control plane information are unknown.In this example, the stack includes three FEC-Query entriescorresponding to labels 17001, 17004, and 20001. In many cases, the toplabel in the TFS Sub-TLV will be available in the IGP control plane ofthe initiator (e.g., if they belong to same domain). As an optimization,the initiator may use reverse label lookup (by using the top label andsearch for the FEC details in IGP database) and populate the top Sub-TLV56 in the TFS.

In this example, vPE1 includes the FEC details of Spine1 as top FEC 56(Spine1-Prefix-SID). All remaining FECs in the target FEC stack arepopulated as FEC-Query 58. The TTL (time-to-live) of the label stack isset to sequentially increment from 1. As shown in FIG. 5, top label20003 includes a TTL=1. Each subsequent trace request originated by theinitiator includes a TTL that is set successively to 1, 2, and so on.

FIG. 6 illustrates a trace response 60 in which ToR1 validates the topFEC (Spine1-Prefix-SID) and replies to the trace request shown in FIG.5. In one example, the reply may be generated and transmitted inaccordance with RFC 4379 (referenced above).

FIG. 7 illustrates contents of a second trace packet transmitted frominitiator node vPE1. The trace request shown in FIG. 7 is similar to thefirst trace request shown in FIG. 5, except the TTL is now set equal to2. The TFS Sub-TLV includes the FEC 56 for Spine1 and the threeFEC-Query entries 58 (as described above with respect to FIG. 5). Asshown in FIG. 7, when the transit node (Spine1) replies with POP, theresponder will include the FEC details of the associated label in thelabel stack since the underlying FEC in TFS is “FEC-Query”. In responseto the trace packet, Spine1 replies and includes the FEC details of thenext label (Label=17001) in the response 70 (Next-FEC=PE1-Prefix-SID).Spine1 may reply with FEC Stack Change (e.g., as per RFC 6424 or othersuitable method). As may be noted, the replying node (Spine1) and thenode that actually validates the FEC (PE1) are different, therefore anydiscrepancy should be detected.

As shown in FIG. 8, the initiator (vPE1) next generates a trace packetwith an FEC stack including the FEC details 86 received from theresponder (Spine1) and two FEC-Query entries 88. The TTL is set equal to3 and the trace packet is processed at PE1 in the MPLS cloud 14 as shownin FIG. 9.

Referring now to FIG. 9, PE1 replies with FEC-Stack-Change (POP) intrace response 90. While replying, PE1 includes FEC details of the nextlabel (17004 corresponding to PE4) due to the FEC-Query. The FEC detailsfor PE4 are shown in the response as Next-FEC=PE4-Prefix-SID.

The initiator (vPE1) then POPs the top FEC-TLV from the Target FEC stackand replaces the FEC-Query with the FEC information received in theresponse (PE4-Prefix-SID), as shown in FEC stack of FIG. 10. Theinitiator also increments the TTL by one so that it is now equal to 4.

Referring now to FIG. 11, P2 (for TTL=4) and P3 (for TTL=5) validate thetop FEC and reply back with a trace response 110 (e.g., in accordancewith RFC 4379, referenced above).

As shown in FIG. 12, the next trace request generated by the initiatornode vPE1 comprises TFS Sub-TLV 125, which includes the previouslyreceived FEC details for PE4 (PE4-Prefix-SID) and one FEC-Query forremaining label 20001 (corresponding to vPE2). The TTL is now set to 6by the initiator. PE4 replies with trace response 120 withFEC-Stack-Change (POP) and includes FEC details of the next label(Next-FEC=vPE2-Prefix-SID) due to the remaining FEC-Query in the FECstack transmitted in the trace request.

The initiator then POPs the top FEC TLV from the Target FEC Stack andreplaces the first FEC-Query with the FEC received in the trace responsein FIG. 12 (vPE2-Prefix-SID) to generate the next trace request 135(FIG. 13). The TTL is incremented to 7.

As shown in FIG. 14, Spine2 (for TTL=7) and ToR2 (for TTL=8) validatethe top FEC in the trace request of FIG. 13 and reply back with LSP PingReply and FEC Validated (e.g., per RFC 4379) in response 140.

Node vPE2 (for TTL=9) can now validate the top FEC in response to tracerequest 155 and reply back with Egress for FEC in trace response 150, asshown in FIG. 15. As previously noted, vPE2 may generate a traceresponse (echo reply) in accordance with RFC 4379.

FIGS. 16-19 illustrate a process for Target FEC Stack based FEC queryutilizing Ping mode, in accordance with one embodiment. In Ping mode,the initiator sets the TTL (time-to-live) of bottom label as 1 and setsthe Target FEC Stack Sub-TLV with FEC-Query. The responder replies backwith the FEC of the last label and the initiator uses the same for LSPPing.

FIG. 16 illustrates initiator behavior for ping mode. Node vPE1 sets FECin Target FEC Stack 166 as FEC-Query and sets the TTL of bottom labelto 1. The initiator (vPE1) includes an FEC-Query for the FECcorresponding to the destination of the segment.

As shown in FIG. 17, PE4 replies with trace response 170 including FECdetails of next label (Next-FEC=vPE2-Prefix-SID) due to FEC-Query in thetrace request of FIG. 16.

Initiator node vPE1 can now set FEC in Target FEC Stack 186 to vPE2-SID(FIG. 18) based on the FEC details received in trace response 170 (FIG.17) and transmit the trace request to vPE2.

As shown in FIG. 19, egress node vPE2 replies with a trace response 190providing FEC validation for the LSP.

It is to be understood that the topology and processes described aboveand shown in FIGS. 5-19 are only examples and that the embodimentsdescribed herein may be implemented in other topologies and use otherprocesses, protocols, or packet formats without departing from the scopeof the embodiments.

Although the method and apparatus have been described in accordance withthe embodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations made to the embodiments withoutdeparting from the scope of the invention. Accordingly, it is intendedthat all matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A method comprising: generating a first tracerequest at an initiator node configured for segment routing, said firsttrace request comprising a query for FEC (Forwarding Equivalence Class)information; transmitting said first trace request on a path comprisingat least one node wherein FEC details for the node are unknown by theinitiator node; receiving a response to said first trace requestcomprising the FEC information; transmitting a second trace request withthe FEC information; and receiving a response to said second tracerequest providing FEC validation.
 2. The method of claim 1 wherein saidsecond trace request is transmitted on an LSP (Label Switched Path) overat least one MPLS (Multiprotocol Label Switching) tunnel.
 3. The methodof claim 1 wherein said FEC validation provides end-to-end validation ofan LSP (Label Switched Path).
 4. The method of claim 1 wherein saidfirst trace request comprises a Target FEC Stack Sub-TLV containing saidFEC query.
 5. The method of claim 4 wherein the Target FEC Stack Sub-TLVcomprises said FEC query for each label in a label stack with unknownFEC details.
 6. The method of claim 4 wherein transmitting said secondtrace request comprises replacing said FEC query in the Target FEC StackSub-TLV with the received FEC information.
 7. The method of claim 1wherein transmitting said second trace request comprises incrementing aTime-to-Live (TTL) by one.
 8. The method of claim 1 wherein transmittingsaid first trace request comprises setting a Time-to-Live (TTL) of abottom label to one and inserting said FEC query into said first tracerequest for an egress node of an LSP (Label Switched Path).
 9. Themethod of claim 1 wherein the path comprises at least two stitched LSPs(Label Switched Paths).
 10. An apparatus comprising: an interface fortransmitting packets on an LSP (Label Switched Path); a processor forgenerating a trace request at an initiator node configured for segmentrouting, the trace request comprising a query for FEC (ForwardingEquivalence Class) information, transmitting said trace request on apath comprising at least one node wherein FEC details for the node areunknown by the initiator node, and processing a response to said tracerequest comprising the FEC information; and memory for storing the FECinformation.
 11. The apparatus of claim 10 wherein the processor isfurther configured to transmit a second trace request with the FECinformation and process a response to said second trace requestproviding FEC validation.
 12. The apparatus of claim 10 wherein theprocessor is further configured to transmit the FEC information to acontroller for use by the controller.
 13. The apparatus of claim 10wherein said trace request comprises a Target FEC Stack Sub-TLVcontaining said FEC query.
 14. The apparatus of claim 13 wherein theTarget FEC Stack Sub-TLV comprises said FEC query for each label in alabel stack with unknown FEC details.
 15. The apparatus of claim 10wherein the processor is configured for transmitting a second tracerequest for use in FEC validation and incrementing a Time-to-Live (TTL)in said second trace request by one.
 16. The apparatus of claim 10wherein transmitting said trace request comprises setting a Time-to-Live(TTL) of a bottom label to one and inserting said FEC query into saidtrace request for an egress node of the LSP.
 17. Logic encoded on one ormore non-transitory computer readable media for execution and whenexecuted operable to: generate a first trace request at an initiatornode configured for segment routing, said first trace request comprisinga query for FEC (Forwarding Equivalence Class) information; transmitsaid first trace request on a path comprising at least one node whereinFEC details for the node are unknown by the initiator node; process aresponse to said first trace request comprising the FEC information;transmit a second trace request with the FEC information; and process aresponse to said second trace request providing FEC validation.
 18. Thelogic of claim 17 wherein said second trace request is transmitted on anLSP (Label Switched Path) over at least one MPLS (Multiprotocol LabelSwitching) tunnel.
 19. The logic of claim 17 wherein said FEC validationprovides end-to-end validation of stitched LSPs.
 20. The logic of claim17 wherein said first trace request comprises a Target FEC Stack Sub-TLVcontaining said FEC query.